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From what I understand, prior to the launch, the relief valves are open to allow some of the boiling LOX to evaporate and be replaced with fresh, cold one (we can see oxygen evaporating - and "bursting", from time to time - from the rocket). When some (unknown to me) condition is met, the valves are closed and tanks are pressurized with helium. If someone could confirm or clarify that this is correct, it would be great, because I suspect I'm missing something here.

During the NROL-76 launch, we heard the "pressing for flight" announcement for Stage 2 at T-40s, and for Stage 1 at T-20s.

However, the time of closing these valves vary from launch to launch. For example, during the CRS-7 mission, S1 valves were closed at T-2:20 and S2 at T-50s. During Orbcomm-2 mission, both tanks were said to be pressing for flight at approximately T-1:00.

What is the reason these times vary from launch to launch? Is it because of the different LOX temperatures?

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  • $\begingroup$ This may be SpaceX's new helium loading procedure after Amos-6 $\endgroup$ – Jake Blocker May 4 '17 at 11:44
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I got an answer to this on Reddit SpaceX community.

"Pressing for flight" is indeed closing the pressure relief valve and opening a helium valve to fill the remaining headspace above the LOX with helium. Because the engines are not started at this point this is cold helium but once the engines start the helium is heated in a heat exchanger so the headspace is filled with hot helium to reduce the mass of helium required.

SpaceX are continuously varying the propellant loading procedures to maximise the mass that can be loaded into fixed size tanks. If they are closing the vent valves later then it implies that they have reduced the head space to fit more propellant in. In that case the evaporating LOX will overpressurise the smaller volume more quickly so less time is available between closing the valve and the point at which the engines fire reducing the propellant load.

S2 only starts around 140 seconds after the S1 engines so it is pressurised as late as possible consistent with getting the tank to flight pressure to stiffen it up at lift off.

source

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